Prior Art
An injection molding or die casting tool for a part consists generally of two opposing die insert halves--fitted in a larger frame and representing the negative version of the part that is to be produced. These two halves are known respectively as the cavity, female or convex half-representing the outer surfaces of the part; and the core, male or standing half-representing the inner details of the part. When the two halves are closed in an injection molding process, plastic or related material is molded into the cavities formed by the two halves to produce the desired part shape.
The standard method for the fabrication of a molding tool begins with the splitting of a three dimensional CAD representation into the two above mentioned cavity and core halves, and using the computer database to create a positive or male version of the parts. The positive version of the part is normally machined or ground as a set of carbon or copper electrodes for each mold half, and these set of electrodes are then used to burn a negative or female representation of the parts into a block of steel, one for the cavity half of the tool, the other for the core.
These two tooling halves can then be mounted on a standard injection molding machine to mold the actual parts from plastic, metal, ceramic or composite material formulations. Hard tooling for injection molding such as described above, is also used to produce patterns for the investment casting process as well as several powder metallurgy processes.
This process of machining a positive version of the part out of carbon material or in some cases of actually directly machining the negative or mirror image of the part into each mold half is very time consuming and expensive. Adding to the time and cost of producing the tools, is the detail work needed to incorporate water lines for cooling the mold and the fabrication of ejector pins, ejector holes and slide and wear components. Depending on the complexity and size of a tool, fabrication time can take from six weeks for a simple tool to an average of twenty-four weeks for a larger more complex tool. One application of the instant invention is to provide an alternate means to manufacture permanent tooling beyond the established technologies of machining & grinding.
An alternate means of fabricating mold cavities without using machining or grinding, has been to build the features from several different mold components. This saves time with respect to fabricating one large carbon electrode, but does require a great deal of detail work. A second application of the instant invention is then to provide a means to produce these mold components by reducing the amount of detail work needed to use them.
Because the established tooling technologies require a great deal of time and expense, a series of rapid prototyping technologies have been developed with which design engineers can evaluate form and function. The main issue is that new product designs must often undergo changes to allow all the component parts to be integrated and to function adequately and to specifications. The only way this can be done is to actually produce and assemble the parts in order to obtain the database for possible improvements and design changes. This requires either the manufacture of single prototype parts or the fabrication of rapid prototype molds that can process a series of parts. The third application of the instant invention is to provide a means to produce rapid prototype tooling.
Single prototype parts can be produced by technologies such as stereolithography, where a computer guided laser converts a liquid of a specific composition into a solid three dimensional part. Other solid modeling technologies using fundamentally the same principle, but different forming mediums, include laminated object manufacturing(LOM)--that uses fine paper to form the object, selective laser sintering(SLS)of powders, precision metal spraying and others including CNC machining of various materials
The problem for single prototypes remains however, that while time is gained by skipping the construction of a hard tool; and cost reduced by obtaining parts for functional testing and evaluation, this testing is necessarily limited in scope. The prototype processes can produce only a limited number of parts, and increasing the quantities can outweigh the cost savings obtained by avoiding the construction of a hard tool. This fact highlights the importance of developing methods for the rapid fabrication of prototype tools. Many of these methods have been based on the fundamental single prototyping technologies.
For example, U.S. Pat. No. 5,458,825 describes the use of stereolithography to directly produce blow molding tooling for rapid container prototyping. This method of direct tooling, so called because a pattern is not required in the building of a mold, can produce tools of high accuracy but limited durability, so the volume runs are short. One of the issues is that the choice of materials for the stereolithographic process, referred to as SLA(stereolithography apparatus), is limited, and these materials have to be able to withstand higher molding temperatures to accommodate a wider range of plastics for sampling. Another means of duplicating the construction of the cavity and die halves of a molding tool as described in U.S. Pat. Nos. 5,435,959 and 5,580,507, involves using a prototype of the part itself inserted into a container so that the relevant features can then be engulfed by a resilient plastic material. The material is removed from that container, so that subsequently the sample itself can be separated from the resilient material leaving a cavity of that feature. The process is then repeated for the other feature to produce another mold half.
A variation of the above mentioned process uses an RTV (room temperature vulcanizing) silicone that is vacuum cast and subsequently cured around patterns generated from any of the solid modeling technologies such as SLA, SLS (selective laser sintering), LOM (laminated object manufacturing), CNC machining and others. These tools can run up to 40 prototype parts.
Variations of the above mentioned processes beginning with the 3-D CAD files, creating patterns from stereolithography or other materials and using these patterns to cast reinforced resins or plastic, are all categorized under the term "soft" tooling, due to the fact that the tooling wears out easily. "Soft" tools by definition then, can only be used as a means of rapid prototyping a limited number of parts. The short-life aspect of this tooling has been addressed by other developing technologies geared towards producing the desired "hard" tools made out of metal, and hence having more durability.
For example, a recent innovation, described in U.S. Pat. No. 5,641,448, takes the "soft" tooling produced by the any of the above mentioned solid modeling technologies, and selectively deposits layers of nickel around the inner mold surfaces. The nickel coated mold is then fitted into a base for the injection molding operation. This process does harden the tool to increase the tooling life, nonetheless, molding parameters must be controlled towards the lower end of the molding pressures to maintain the nickel deposited layers intact.
Another method of manufacture described in U.S. Pat. No. 5,189,781, takes the cavity and core patterns directly produced from stereolithography or a related solid modeling technology, and proceeds to thermal spray metal on the substrates. The next step involves the separation of the patterns from the metal substrates, leaving the cavity and core mold halves exposed. The strength of these thermally sprayed molds is a function of the thickness of the thermally sprayed metal, hence they tend to be fragile in nature.
Other methods addressed to the rapid fabrication of "hard" tools for prototype and production purposes, include the use of investment casting as described in U.S. Pat. No. 4,220,190, where the investment cast shell serves as a means to form the functional cavity surfaces when the metal alloy is cast. In a variation on this, SLA patterns are being used for the direct casting of the injection tooling molds. The main issue with this fabrication method has been the inherent surface quality of the casting and the amount of work required to bring the cast mold or die halves to specifications for use in injection molding tools.
Methods of thermal spraying of metal have been developed to directly produce prototype parts and more recently to form "hard" molds, die and tools as described in U.S. Pat. No. 5,609,922. The patterns in this case are support members constructed not only to form the desired shape of a cavity or a core, but also to promote optimum heat exchange properties for the thermal spray deposition process. The tools have the same issues of fragility as other thermal spray methods that use patterns from stereolithography as a base for the thermal spray deposition process.
Powder metallurgy has been applied towards the construction of molds and mold components due to the fact that powders can conform to the shape of any given pattern when they are flowed in. Variations in the application of the process can be identified by the way the powders are consolidated so they can maintain the desired shape. For the purposes of forming complex metal molds, the advantages of powder metallurgy lie not only in the forming of complex shapes facilitated by the flow of powders, but also by the fact that a great deal of material waste can be avoided by processing net shape or near net shape molds when compared to the other metal working processes.
A means of forming the die cavities through the use of conventional powder metallurgy is described in U.S. Pat. No. 4,327,156. The practice of this invention involves flowing in refractory powders around a flexible rubber mold that has been previously conformed from a replicating master. To keep the powders in place, a binder is mixed with the powders and molded or compressed into shape, followed by a curing period to allow the binder to harden and hold the part shape. The next step is remove the cavity or core mold, and to burn off the binder in an oven once it has accomplished its purpose, thereby leaving a porous metal skeleton that can be closed off by infiltrating a low melting point metal such as copper. This method does provide "hard" tooling that will last longer than the "soft" tooling of the other rapid prototyping technologies, and introduces the use of powder metallurgy as a means to form the "hard" tooling.
A variation of this process as described in U.S. Pat. No. 5,507,336, casts a ceramic compound over a pattern to form the cavity or core half. The procedure is to take the cavity impression on the ceramic casting and place in a tubular container so that loose metal powder can be poured into the container. Instead of binding the powders together with a binder as in U.S. Pat. No. 5,507,336, the whole tubular container is placed in an oven and a low melting metal such as copper is melted over the powder to bind the whole shape. The next step is to remove the original ceramic pattern to leave exposed the desired cavity or core mold half, which can then be assembled into a complete tool for injection molding.
Improvements have been commercially incorporated into this methodology by coating the fine metal powders by a proprietary polymer and selectively laser sintering the coated powders around a given pattern. In this case the laser serves to fuse the polymer and holds the shape of the part, thereby eliminating the need for any tubular shaped container to hold the powders together. This "green" part is subsequently impregnated with a low-melt binder system and heated in an oven before sintering at higher temperatures to provide a metal skeleton, that in the final steps is infiltrated with copper. This process is know as "Rapid tool-Long Run(LR)" and is practice by DTM Corporation in Austin, Tex.
The above mentioned approaches have addressed the issues of tool longevity by using powder metals to form "hard" metal dies. Though the resulting molds are more permanent in nature, there are two main issues which prevent these tools from being considered permanent hardened tools. The first is that the tools are difficult to polish due to the coarse nature of the base powders. This means that the surface finish on parts produced from these tools may not be adequate. The second issue is that the high copper content- necessary to close the porosity in the initial metal skeletons- reduces the attainable hardness of the composite to about Rockwell B75, which is softer that similar tools machined from aluminum. Tool life and wear resistance remains a major issue when compared to tools manufactured from conventional methods that can be hardened above Rockwell C60.
A recent application of powder metallurgy as a method for producing dies is described in U.S. Pat. No. 5,435,824. It applies hot isostatic compacting to develop a fully dense mold and die block that does not need to be copper infiltrated to achieve full density. Hot isostatic compacting consists of using a rubber container which has the general shape desired, to hold the powders together while they are compacted into shape by high pressures. The process includes removing the rubber container once the mold can hold its shape, and then heating or sintering the "green" article in a furnace to consolidate the metal powders. Several alloys can be processed from this method that can attain harnesses equivalent to those of the wrought materials commonly applied in the toolmaking process. The main issue with using this process for the construction of molds and mold components, is that the method is inherently limited in the complexity of components that it can reproduce as well as issues having to do with dimensional accuracy, since compmolds and dies occurs in molds and dies occurs in a directional basis.
Each of the above mentioned inventions has improved the development process by reducing the elements of time and cost, yet each has issues that detract from its adoption as production and extended run tools.
Those technologies that use "soft" tooling achieve only a limited number of parts for evaluation. This is due to the fact that all plastic tools made of resilient materials such as nylon and reinforced fibers tend to wear away, so that reproducibility is compromised.
When improvements are accomplished by the rapid fabrication of metal molds, these then have issues relating to surface finish that may detract from form and function evaluations on certain parts applications. In addition to this dimensional tolerances of the resulting tools may vary because the copper infiltration process causes some expansion of the mold, or the method of compaction provides a directional bias, as in hot isostatic compacting. None of the present state of the art rapid die fabrication technologies have been able to produce molding dies from a hardenable tool steel, related metal alloy or hardened material that can approach the metallurgical, surface finish quality and complexity levels of the conventional die fabrication methods.